In the semiconductor industry there is a continuing trend toward increasing device densities, throughput and yield. To increase device densities there have been, and continue to be, efforts toward scaling down semiconductor device dimensions (e.g., at sub-micron levels). In order to accomplish such densities, smaller feature sizes and more precise feature shapes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry, such as corners and edges, of various features. To increase throughput, the number of required processing steps can be reduced and/or the time required for those processing steps can be reduced. To increase yield, which is the percentage of finished products leaving a fabrication process as compared to the number of products that entered the fabrication process, control and/or quality of individual fabrication processes can be improved.
Semiconductor fabrication is a manufacturing process employed to create semiconductor devices in and on a wafer surface. Polished, blank wafers come into semiconductor fabrication, and exit with the surface covered with large numbers of semiconductor devices. Semiconductor fabrication includes a large number of steps and/or processes that control and build the devices—basic processes utilized are layering, doping, heat treatments and patterning. Layering is an operation that adds thin layers to the wafer surface. Layers can be, for example, insulators, semiconductors and/or conductors and are grown or deposited via a variety of processes. Some common deposition techniques are chemical vapor deposition (CVD), evaporation and sputtering. Doping is the process that adds specific amounts of dopants to the wafer surface. The dopants can cause the properties of layers to be modified (e.g., change a semiconductor to a conductor). A number of techniques, such as thermal diffusion and ion implantation can be employed for doping. Heat treatments are another basic operation in which a wafer is heated and cooled to achieve specific results. Typically, in heat treatment operations, no additional material is added or removed from the wafer, although contaminates and vapors may evaporate from the wafer. One common heat treatment is annealing, which repairs damage to crystal structure of a wafer/device generally caused by doping operations. Other heat treatments, such as alloying and driving of solvents, are also employed in semiconductor fabrication.
Patterning is a series of steps that results in the removal of selected portions of surface layers. After removal, a pattern of the layer is left on the wafer surface. The material removed can be, for example, in the form of a hole in the layer or a remaining island of the material. The patterning transfer process is also referred to as photomasking, masking, photolithography or microlithography. The actual subtractive patterning (i.e., removal of material from the surface film) is performed by plasma etching. The goal of the patterning process is to create desired shapes in desired dimensions (e.g., feature size) as required by a circuit design and to locate them in their proper location on the wafer surface. Patterning is generally considered the most important of the four basic processes.
Generally, a process control system is employed to perform one or more patterning or lithography processes. The process control system controls various parameters of the process such as, development time, resist flow and the like. Additionally, the process control system monitors characteristics such as etch rate, dimensions, features size and the like in order to determine whether fabricated devices are acceptable. The patterning or lithography processes are required to be performed and generate results within a historical tolerance, referred to as control limit(s) (e.g., +/−10%). However, these control limit(s) fail to account for specific design requirements or product yields and can thus permit unacceptable devices to be deemed acceptable and acceptable devices to be deemed unacceptable.